(1) Field of the Invention
The present invention relates generally to a bias circuit and, more particularly, to a bias circuit for generating bias voltages, i.e., constant voltages, which may be, for example, supplied to the inputs of a pre---amplifier and to the inputs of an input stage of a power amplifier, in an audio-amplifier.
(2) The Prior Art
In a pre-amplifier or an input stage of a power amplifier, in order to stabilize its operation, it is necessary to apply stable bias voltages to the inputs of the pre-amplifier or the like. In addition, in recent years, the pre-amplifier or the like has been manufactured by using semiconductor technology, especially, integrated circuit technology, so the bias voltages applied to the pre-amplifier or the like are relatively low. Therefore, it is necessary to apply low and stable bias voltages to the pre-amplifier or the like.
One prior art bias circuit for generating bias voltages which may be, for example, applied to the inputs of a pre-amplifier or the like, is composed of diodes whose forward voltages serve as bias voltages. However, the change of the voltage applied to the diodes varies the forward resistance of the diodes, i.e., varies the forward voltages of the diodes. Therefore, the bias voltages generated from the bias circuit formed by diodes are unstable with respect to the change of the power voltage supplied to the bias circuit.
Another prior art bias circuit is composed of Zener diodes whose Zener voltages serve as bias voltages. The bias voltages generated from the bias circuit formed by Zener diodes are stable with respect to the change of the power voltage supplied to the bias circuit. However, it is difficult to generate a low bias voltage, for example, less than 5 volts.